Disposable air filters of the types used in home heating systems to filter particles from air traveling through air plenums typically comprise (1) a sheet of filter media shaped (e.g., pleated, corrugated, or planar) so that portions of opposite major surfaces of the sheet of filter medial are disposed to define parallel planes along those opposite surfaces intended to be placed normal to the direction of movement of air through the filter; and (2) a rectangular frame of folded and adhered together chip board portions around a peripheral edge of the sheet of filter media. Typically, the frame of the air filter is slid edgewise into a U-shaped metal filter mounting channel that extends around three or four sides of the plenum. The channel then closely engages the frame of the air filter on at least three sides to position the sheet of filter media in the air filter transversely across the air plenum. Particulate in air traveling through the plenum is collected in the filter media. As the filter media fills with particulate, it provides increased resistance to the passage of air resulting in increased air pressure against one face or side of the filter. Engagement between the channel and the frame of the filter should prevent deformation of the filter as a result of this pressure, for if it does not, unfiltered air can pass around the edges of the deformed filter.
FIGS. 1 and 2 of the drawing illustrate one commercially available prior art disposable air filter 12 including a chip board frame 10 around a pleated sheet 18 of filter media, whereas FIGS. 3 and 4 of the drawing illustrate another commercially available prior art disposable air filter 13 including a chip board frame 11 around a pleated sheet 19 of filter media. The chip board frames 10 and 11 in both of those filters 12 and 13 include elongate edge wall portions 14 and 15 respectively disposed in rectangles and generally at right angles with respect to parallel planes 16 and 17 respectively defined by parts of the pleated sheets 18 and 19 of filter media, which edge wall portions 14 and 15 extend along, across, and entirely around rectangular peripheral edges 20 and 21 respectively of the sheets of filter media 18 and 19. Those chip board frames 10 and 11 also include elongate retaining wall portions 22 and 23 respectively that extend from the opposite edges of the edge wall portions 14 and 15 for short distances (e.g., 1 inch or 2.54 centimeters) along the opposite major surfaces of the pleated sheets of filter media 18 and 19. Parts 24 and 25 respectively of those retaining wall portions 22 and 23 opposite each other and spaced from the edge wall portions 14 and 15 are attached by means such as a suitable adhesive to the sheets of filter media 18 and 19 (the end of the pleats in the filter media 18 and 19 being flattened between those parts 24 and 25 of the retaining wall portions 22 and 23), and end parts 26 and 27 respectively of the retaining wall portions 22 and 23 at the corners of the filter 12 or 13 overlap and are attached by means such as a suitable adhesive. In the prior art disposable air filter 12 illustrated in FIGS. 1 and 2 that is of the type sold by American Air Filters, Louisville, Ky., under the trademark "Dirt Demon" those retaining wall portions 22 both extend from the edges of the edge wall portions 14 at included angles significantly less than a right angle with respect to the edge wall portions 14 (i.e., angles of about 60 degrees) so that the cross section of the frame 10 (see FIG. 2) is generally in the shape of an equilateral or isosceles triangle. In the prior art disposable air filter 13 illustrated in FIGS. 3 and 4 that is of the type sold by Minnesota Mining and Manufacturing Company, St. Paul, Minn., under the trade name "Filtrete"(t.m.) High Performance Clean Air Filter, those retaining wall portions 23 on one side of the filter 13 extend from corresponding edges of the edge wall portions 15 at about right angles with respect to the edge wall portions 15, and the retaining wall portions 23 extending from the edges of the edge wall portions 15 on the other side of the filter 13 extend at angles significantly less than a right angle with respect to the edge wall portions 15 (i.e., an angle of about 45 degrees) so that the cross section of the frame 11 (see FIG. 4) is generally in the shape of a right-angled triangle. The frames 10 and 11 in these air filters 12 and 13 are of a type called "pinch frames" in the air filter industry, and only extend around peripheral portions of the sheets 18 and 19 of filter media, leaving totally exposed and unsupported both sides of all but those peripheral portions of the sheets 18 and 19 of filter media that are received in the frames 10 and 11.
FIGS. 5 and 6 of the drawing illustrate another prior art disposable air filter 28 having a frame 29 of folded and adhered together chip board portions that is sold by Minnesota Mining and Manufacturing Company, St. Paul, Minn., under the trade name "Filtrete"(t.m.) Micro particle air filter. The frame 29 of that air filter 28 is of a type called a "box frame" in the air filter industry, and more fully encloses a pleated sheet 30 of filter media than do the "pinch" frames 10 and 11 of the air filters 12 and 13 illustrated in FIGS. 1 through 4. That frame 29 includes two essentially identical parts 31, each of which parts 31 includes elongate edge wall portions 32 disposed in a rectangle and generally at right angles with respect to two parallel planes 33 defined by parts of the pleated sheet 30 of filter media, which edge wall portions 32 extend along, across, and entirely around a rectangular peripheral edge 34 of the sheet 30 of filter media. Each part 31 of the frame 29 also includes a side wall portion 35 extending from corresponding edges of the edge wall portions 32 entirely across the sheet 30 of filter media along and parallel to one of the two parallel planes 33 defined by parts of the major surfaces of that sheet 30. That side wall portion 35 includes a continuous retaining wall part 36 extending a short distance (e.g., 1 inch or 2.54 centimeters) from corresponding edges of the edge wall portions 32 at about right angles with respect to the edge wall portions 32, and intersecting grid-like parts 37 extending between oppositely disposed locations along the retaining wall part 36 and defining large triangular or diamond shaped openings 38 through the side wall portion 35 that afford free passage of air through the side wall portion 35. The two parts 31 are disposed with their side wall portions 35 on opposite sides of the sheet 30 of filter media and major surfaces of their edge wall portions 32 adjacent and adhered together so that the adhered together edge wall portions 32 and the retaining wall parts 36 extending from them provide a generally U-shaped receptacle for a portion of the sheet 30 of filter media adjacent its periphery, in which U-shaped receptacle that portion of the sheet 30 of filter media is adhered.
While the frames 10, 11 and 29 of the disposable air filters 12, 13, and 28 described above and illustrated in FIGS. 1 through 6 of the drawing can prevent deformation of the filter under normal operating conditions, it has been determined that under extreme test conditions reported below they can be caused to deform, thereby allowing unfiltered air to pass around the edges of the deformed filter 12, 13, or 28.